The present invention relates to light curtains, in particular safety light curtains, for monitoring a protective field. Furthermore, the present invention relates to optical units which are part of such a light curtain.
Generally, light curtains detect the movement or intrusion of objects into guarded zones, and more particularly, provide protection for human operators who are working with machines or other industrial equipment.
Light curtains employing infrared or visible light beams are used to provide operator safety in a variety of industrial applications. Light curtains typically are employed for operator protection around machinery, such as punch presses, brakes, molding machines, automatic assembly equipment, coil winding machinery, robot operation, casting operations and the like. Conventional light curtains typically employ light emitting diodes (LED) mounted at spaced positions along a transmitter bar at one side of the guard zone and phototransistors (PT), photodiodes or photoreceivers mounted along a receiver bar at the opposite side of the zone. The LEDs transmit modulated infrared light beams along separate parallel channels to the PTs at the receiver bar. If one or more beams are blocked from penetration by an opaque object, such as the operator's arm, a control circuit shuts the machine down, prevents the machine from cycling, or otherwise safeguards the area.
Usually, safety light curtains comprise two optical units (called bars, sticks, or strips), which are formed as two different constructional units, one of the optical units having the functionality of an emitter and one of a receiver. This dedicated architecture of an emitter and receiver, however, has several drawbacks, for instance the fact that the fabrication costs are high, because each type of optical unit has to be fabricated differently. Consequently, there exist concepts that use an architecture wherein each optical unit has a transceiver unit carrying a plurality of light emitting elements and light receiving elements and at least one separate detachable plug-in module. The first and second transceiver units are identically built, whereas the first and second plug-in modules differ from each other and thus define the functionality of the respective optical unit. For instance, the plug-in module differentiates an optical unit as the emitter with, for instance, the test input, or as the receiver with, for instance, the output signal switching devices, OSSD.
Such a modular architecture is for instance proposed in the European patent application EP 11162263.5 and allows a very cost-effective fabrication, because the transceiver modules are identically built and, furthermore, can be applied in a very flexible way for a multitude of applications and system configurations.
However, this modular transceiver bar configuration is not compatible with laser alignment techniques that employ one laser module as a radiation emitter at each stick, but at different locations for the dedicated receiver and transmitter bar, respectively, as this is for instance shown in the published European patent EP 0889332 B1.
In order to provide an alignment system for systems based on transceiver bars according to the European patent application EP 11162263.5, one possible solution could therefore be seen in providing two laser modules for each transceiver bar. This concept, however, increases the costs due to the additional laser emitter for each transceiver bar.
The object underlying the present invention is to provide a light curtain and an optical unit for a light curtain, which can be fabricated in a particularly cost-effective way, and allow for an accurate alignment and synchronization.
This object is solved by the subject matter of the independent claims. Advantageous embodiments of the present invention are the subject matter of the dependent claims.